Was it possible that breastfeeding BabyC could affect her level of fitness as a teenager? I was intrigued.

The study tested cardiovascular fitness on a stationary bike in about 2000 children and teenagers from Sweden and Estonia. The kids’ mothers were asked to recall if they breastfed their children, and if so, for how long (<3 months, 3-6 months, or >6 months). Children that were fed a mix of breast milk and formula for any period were eliminated from the study.

The researchers found that breastfed kids had about 5% greater cardiovascular fitness than those fed formula, and fitness was highest in children who had been breastfed exclusively for at least 3 months. This finding held true even after the researchers adjusted for country, gender, age, puberty, BMI, birth weight, physical activity level, maternal BMI and maternal education.

A 5% increase in cardio fitness may not seem like much, but it is actually rather impressive when you consider all the other factors that are involved. Genetics are thought to explain about 50% of fitness, and body weight and activity level (how much aerobic activity a person routinely does) also play a big role.

As often happens when I read journal articles, this study led me to another published in 2010. Among more than 2500 teenagers from around Europe, Enrique Artero and colleagues found a significant correlation between how long they were breastfed as infants and how far they could long jump. Boys that were breastfed for 6 months or longer had an 11-cm edge over formula-fed boys, and girls had a 7-cm edge. On the other hand, there was no association between breastfeeding and speed in the 20-meter shuttle run. (That sure brings back memories from middle school!) Like the previous study, these data were adjusted for factors like the children’s physical activity and body composition and parental weight and education.

So breastfeeding my child means she’ll be better at both cycling and the long jump?!

Not so fast. You know I’m not going to report the results of new research without talking about its limitations. Both of these studies are retrospective, cross-sectional studies. They looked at kids that were breastfed and those that weren’t and compared their physical fitness. In an ideal world, if you wanted to know if breastfeeding was related to physical fitness later in life, you would enroll a bunch of pregnant women and assign them to either the breastfeeding group or the formula-feeding group. Then, 10-15 years later, you would run their kids through physical fitness tests. We all know that this type of prospective, randomized trial will never happen. No mother is going to let a researcher tell her how to feed her baby. Instead, each mother makes that choice herself, and there are many factors that contribute to her choice.

These types of retrospective, cross-sectional studies of breastfeeding always have one big flaw: they simply can’t account for every factor that may be different between breastfeeding and formula-feeding mothers. My guess is that the researchers have only scratched the surface by including maternal BMI and education in their statistical models. What about exercise during pregnancy? Or mom’s nutrition during pregnancy and lactation? How about exposure to cigarette smoke? These are all factors that might be different between breastfeeding and formula-feeding moms. Any of these factors, in addition to breastfeeding, might influence children’s later fitness level by epigenetic mechanisms or more directly, such as by affecting the rate and timing of muscle growth. Research on the benefits of breastfeeding is very hard to do.

As a skeptic and a scientist, I tend to think that this fitness effect is not just about breast milk but probably intertwined with other factors. But as a nursing mom, it is kind of cool to think about. I’ve tried to tell my daughter that the long jump may be in her future, given her 7 cm edge. She doesn’t seem to care. She has been practicing athletic feats during our recent nursing sessions, but they are more yogic in nature. I swear she did a one-legged downward dog the other day without breaking her latch!

Ha! I haven’t seen any studies of breastfeeding and aging yet, but I’m sure they’re coming! It is kind of frustrating, though. The limitations of this type of study design makes you wonder why we keep doing studies like this. I suppose many of these studies come out of datasets from large trials. The researchers have the data and it makes an interesting comparison, but there is no way of knowing if the effect is real.

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Welcome! I'm the author of The Science of Mom: A Research-Based Guide to Your Baby's First Year, published by Johns Hopkins University Press in 2015. I write about science, health, parenting, and nutrition here and for other print and online outlets.
Find more about me at my website: https://alicecallahanphd.com/

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